FIGS. 1 and 2 illustrate prior art DC-DC converters. In FIG. 1 the common terminal of split transformer secondary (output) windings N1 and N2 is connected to a return voltage such as ground while the other terminals are connected to an output terminal 10 through diodes D1 and output inductor or choke L out as shown. An output capacitor is connected between the output terminal and ground so that rectified DC current is provided at 10. The circuit of FIG. 2 is a replica of FIG. 1 except the diodes D1 have been replaced with metal oxide semiconductor field effect transistors (MOSFETS) which are synchronously controlled by controller 18.
The efficiency of all DC-DC converters is dependant upon the voltage drop of the output rectifier(s), e.g., D1 in FIG. 1. As the power density increases(MOSFETS) Q1 and Q2 (FIG. 2) are used instead of the diodes. The MOSFETS' gates are driven in synchronism with a pulse width modulation (PWM) controller 11.
The problem with synchronous rectification for output voltages of 12v or larger is the reverse recovery of the MOSFET's internal body diode in conjunction with the output winding's series leakage inductance. When the MOSFET turns off the current in the series inductance, labeled Ids in FIG. 3a, continues to flow in the body diode and then resonates to a peak negative current—IRR (FIG. 3b). After the reverse recovery period (TR), the negative current in Ls causes a large positive spike across the MOSFET's drain source (Vds). A conventional snubber network R-C (FIG. 3a) dampens this voltage spike at the cost of heavy losses which make the 12v or higher synchronous rectification not practical.
U.S. Pat. No. 6,128,206 (“'206 patent”) to Sun et al discloses a DC-DC synchronous rectifier power converter in which energy resulting for parasitic inductance when the transistor switches (MOSFETS) are turned off is utilized to drive the gate terminals of the switches. While the '206 circuitry may save some energy at low power requirements at high power requirements the additional current supplied to the MOSFETS' gate terminals is wasted.
There is a need to improve the efficiency of synchronous rectifier DC-DC converters at all power settings.